My kitchen range as a long, narrow firebox that's also shallow. About 6 x 6 x 15 originally. And the firebrick liner is made narrower by a about a 1/2 thick layer of very rough, hard build up of burned in clinker.
I've watched vids of how others shake stoves with larger fireboxes and mine won't clear itself if I was that "gentle" with it.
Sometimes it will get "bridging" of not completely burned coal chunks that form in an arch like a church doorway when the softer bottom of the ash pile drops through the grates. I then need to just poke the top to collapse it down onto the grates before adding fresh coal.
Sometimes the ash chunks are too big and there's not enough weight of coal above them to push them down through the grates during shaking. Then I use a poker I made out of hardware store 1/4 rod epoxied into a wooden file handle. The tip is bent at a right angle long enough to reach up through the grates.
If you shake the grates and your not seeing some light coming down through the grates from burning coal, you need to shake harder and/or make a long right angle poker to reach up between the grates and loosen things a bit better.